Playing hardball - improving steel alloy bearings

Materials World magazine
,
4 Dec 2011
Toroidal roller bearing

A new heat treatment process is improving reliability of steel alloy bearings. Phil Burge, from SKF bearings, explains how.

Millions of bearings are sold globally each year forming one of the crucial, unseen components in almost every item of machinery. Although it is just a small part in a big machine, the bearing can make a significant contribution towards energy efficiency by reducing friction and torque, which cuts running costs of the machine. Through ongoing development programmes by manufacturers, the bearing can now provide better levels of performance and increase operating life even in the most challenging environments.

A primary goal in bearing development has been eliminating fracture fatigue in the raceways caused by severe static as well as cyclic loads. One area that has seen significant improvement is the manufacture of highgrade, high-performance steel alloys for bearing rollers and the inner and outer rings. These are the two main components that ensure smooth and efficient rotation while remaining in contact with one another, and it is the toughness and strength of these interacting component materials that determine bearing life expectancy.

Treating for toughness

More than six million tonnes of bearings are produced globally each year, with the vast majority manufactured from steel alloys. However, over the years there have been difficulties in producing a steel alloy that combines high levels of hardness with optimum toughness. Normally, hardened steel is relatively brittle and the presence of stresses created in the manufacturing process can make it unusable in practical applications without additional heat treatment.

This means bearing manufacturers need to develop a steel alloy with optimum mechanical characteristics. This is important, as bearings have to withstand many different operating environments. Any new material must, for example, have adequate hardness and strength to prevent cracks from rolling contact axial loading and provide resistance to chemicals, contaminants and corrosion, while remaining sufficiently ductile during manufacture.

Getting the right balance

Generally, the hardening and toughening process requires steel to be heated fairly slowly to a predetermined temperature and then cooled. Providing the rate of heating has been slow enough for the steel to reach structural equilibrium at its maximum temperature, the rate of cooling determines the final properties of the steel.

Manufacturers of bearing steels have made considerable process improvements in recent years, producing high-grade steels with low levels of impurities, in particular achieving a decrease in oxides. These so-called austenitic steel alloys have a close porosity microstructure, which is suitable for hardening and toughening through further heat treatment, quenching and tempering. 

In an attempt to further improve the internal structure and give the best mechanical properties in relation to the intended bearing application, most steel alloys are now hardened to form either a martensitic or bainitic steel microstructure. A martensitic hardening state is formed by rapid cooling and quenching of austenitic steel in oil and salt, which is then transformed as the temperature decreases. Under the microscope, martensite appears as a mass of uniform needle-shaped crystalline structures that result in high hardness properties. However, martensitic steel can have limited ductility and be prone to tempered embrittlement between the hardened martensitic outer layer and austenite grain boundaries, leading to fracture under certain loading conditions.

This problem has led bearing manufacturers to produce bainitic steels to address the embrittlement associated with martensite steel, although it has been achieved only at the expense of other performance characteristics. Bainitic steels are created by a process called austempering and are characterised by having a needle-like laminated microstructure, which offers higher fracture strength but a lower hardness property, thereby decreasing overall bearing life expectancy.

Bainitic breakthrough

A bainitic steel has now been developed that increases bearing life through a heat treatment process. The outcome is a hardness at least as high as that obtained with martensitic heat treatment or case hardening. The new bainitic steel has extremely low oxygen content and is produced without loss of desirable toughness, crack resistance and the structural strength characteristics typically found with standard bainite structures. This fact is confirmed through wear test results that show resistance of the new bainitic steel is three times greater than steels previously used.

The specially developed heat treatment process hardens the surface of the bearing steel, leaving all other mechanical or physical properties unaffected. In fact, the modified bainitic transformation helps obtain the best properties of the steel. The hardness increases at the same time as the toughness by more than 60% compared with standard banite heat treatment.

This performance is further enhanced by the ability of the new steel to resist stress and fatigue cracks. In fact, micro-fissures that develop can penetrate to a far greater depth before a critical failure occurs, and take longer to develop from initial spall to throughfracture. Compared to conventional martensite steels, the new bearing steel has a crack depth in a typical bearing of up to 150% greater when measured as a percentage of ring wall thickness.

In an operational context, catastrophic bearing failure can be avoided due to the increased strength. The far slower and more predictable rate at which this steel fractures occur provides an opportunity to schedule maintenance activities, thereby maximising mean time between failures and machine uptime.

Further information

Phil Burge, SKF (U.K.) Limited, Sundon Park Road, Luton, Bedfordshire, LU3 3BL, UK. Tel: +44 (0)1582 496433. Email: phil.burge@skf.com